Post-tensioned concrete involves the application of tension to a steel cable strand cast in hardened concrete, followed by anchoring the stretched steel cable strand at the member ends, which produces compressive stress in concrete. This acts to improve the response of a concrete member or structure to loading and reduces deflections and cracking while allowing longer clear spans, thinner slabs, and lighter structures.
Concrete by itself is very strong in compression, but weak in tension, while steel is very strong in tension. To compensate for concrete's natural weakness in tension, post-tensioned concrete imposes a permanent compression load on the structural members. With this type of concrete system, high-strength steel cable strands, often in combination with reinforcing steel bars, are embedded and anchored in concrete. When the concrete has acquired adequate strength, usually three or four days after placement, the tendons are tensioned, (stretched like rubber bands) thereby imposing a compression force on the concrete. These tendons remain stressed throughout the life of the structure, to counterbalance future tension loads.
However, the post-tensioned steel cable strands and bars embedded within the concrete are subject to an increased risk of corrosion caused by the composition of the steel, the stress imposed on the steel, and the ingress of deleterious materials. One method of providing corrosion protection to the steel in the tendons is known as bonded post-tensioning which consists of injecting a hydraulic cementitious grout into the annular space between the duct and the steel in the tendon. If the grout fails to completely encapsulate the steel, whether by incomplete filling of the duct during the grouting operation, entrapment of air in the duct, settlement, or bleeding of the grout after installation, the potential for corrosion to occur in the tendon is increased.
The water in bleed water pockets may evaporate, be reabsorbed by the hydrating hydraulic cementitious grout, or leak from the duct; all resulting in a void being produced next to the steel that allows for corrosion to initiate. The relative anode to cathode area of the corrosion cell, as well as the other factors previously mentioned, have been found to produce rapid corrosion creating structural concerns with numerous structures. Bleed water that does not escape from the bleed water pocket may also freeze and cause rupture of the duct and surrounding concrete. The Post-Tensioning Institute (PTI) defines bleeding in their “Specification for Grouting of Post-Tensioned Structures” as, “The autogeneous flow of mixing water within, or its emergence from, newly placed grout; caused by the settlement of the solid materials within the mass and the filtering action of strands, wires, and bars”. Bleeding is also defined in ASTM C125 as “the autogeneous flow of mixing water within, or its emergence from, newly placed concrete or mortar caused by the settlement of the solid materials within the mass, also called water gain”.
Bleeding is one of the causes of unstable volume and is problematic in non-shrink and post-tensioned grouts. Research has demonstrated that bleeding of grout in ducts for post-tensioning is largely influenced by the interstices formed in the strand by the space between the king wire and the perimeter wires that act as a capillary tube. Gravity causes a pressure head, formed by the difference in elevation between the crown and the trough of the duct profile or the height difference in the lift of the vertical ducts. Temperature, rheology and fluidity play an important factor in bleeding of the combined system.
When the stretched steel cable strands corrode they can break, causing the concrete structure to weaken. As the steel cable strands are not visible or readily accessible, it is very difficult to determine if the steel is corroded and, if so, to what extent. One of the most common techniques to determine if there is a corrosion problem is by observation of cracking in the structure itself, or by drilling into the structure. The act of penetration into the duct to inspect for voids is also likely to lead to the ingress of deleterious materials such as chlorides, oxygen, and carbon dioxide which will then initiate the corrosion process, if the penetration is not permanently resealed. As the steel cable strand is corroded away, the stress on the remaining tendons increases and can lead to structural failure. Repair of failed tendons is very expensive even if structural failure is prevented.
U.S. Pat. No. 5,181,568 to McKown, et al. discloses a method of reducing the water permeability of a subterranean oil bearing formation by a method comprising the steps of: (a) introducing a viscous aqueous pre-polymer composition (polyacrylamide) into the formation which will subsequently form a crosslinked gel therein, and (b) thereafter introducing a hydrocarbon hydraulic cement slurry (hydraulic cement portland, pozzolan, silica slag, or mixtures thereof) into the formation.
U.S. Pat. No. 5,284,513 to Cowan, et al. discloses a hydraulic cement slurry composition comprising: (a) blast furnace slag, (b) a drilling fluid comprising an aqueous phase, clay and salt. The drilling fluid is present in the slurry in an amount sufficient to provide an amount of clay effective to act as a fluid loss control agent. The hydraulic cement slurry also includes an acid functionalized pre-polymer (polyacrylamide) and a crosslinker.
U.S. Pat. No. 5,512,096 to Krause discloses a grouting composition for sealing boreholes or other cavities comprising 90.0–99.99% water swellable clay and 0.01–10.0% gelling agent (polyacrylamide). The grouting composition is mixed with fresh water to provide a low permeability sealing composition.
U.S. Pat. No. 4,015,991 to Persinski, et al. discloses hydraulic cementing compositions and their use in cementing operations which are capable of forming a slurry and which comprise a) dry hydraulic cement; and b) copolymers of hydrolyzed acrylamide and 2-acrylamido-2methylpropane sulfonic acid derivatives, which are used as fluid loss additives for the installation of aqueous hydraulic cement slurries used for cementing wells in subterranean formations.
Examples of copolymer bleeding resistant additives can be found in U.S. Pat. Nos. 3,768,565, 4,015,991, 4,515,635, 4,554,081, and published U.S. application Ser. No. 20010029287, which are all incorporated herein by reference as if written out in full below.
What is needed in the industry is a post-tensioned hydraulic cementitious grout mixture that is resistant to bleeding in order to protect steel cable strands and bars from corrosion caused by water bled out of the mixture, but which has a sufficiently low viscosity to allow for pumpability and ease of placement.